RISONIC 2000 Flow Measurment

Ein Unternehmen der Gruppe BRUGG Schulung RISONIC 2000A company of the BRUGG Group Training RISONIC 2000

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Training

Flow Measurement Fundamentals


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Content

• Flow measurement methods

• Transit time versus Doppler

• Cost Comparison• Basics of ultrasonic transit time

measurement method

• Achievable accuracy with the RISONIC 2000

unit


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Flow Measurement Methods

Level

Measurement

Pressure

Difference

Electro-

magnetic Field

Ultrasonic

Transit Time

p

RIPRESSRIPRESS pressure / level

MPICUFxMPICUFxWRD.1WRD.1

MagMasterMagMaster RISONIC 2000RISONIC 2000


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Flow Measurement Methods Flow Measurement by Differential Pressure

Advantages• Suitable for most gases and fluids• Very common• Costs independent of pipe diameter

Disadvantages• Pressure loss• Sensitive to density and pressure fluctuations• Reduced measuring range (critical lower limit)• Expensive maintenance

Applicable for• Q-measurement insensitive to pressure losses• Turbine flow (Winter-Kennedy method)

Principle:Constricting the pipe diameter increases the velocity and thus forces a reduction in pressure.

Calculating the flow rate from the differential pressure by a function or look-up table.

p


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B

VI

I

Flow Measurement Methods

Electro-magnetic Flow Measurement

v is the mean velocity over the entire cross-section

Electro-magnetic induction B is produced by an alternating or pulsing current I

The inner wall of the cross-section is electrically isolated from the pipe wall. Only the electrodes are in contact with the medium.

Principle:Measuring the induced voltage created by moving the medium through a magnetic field.


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Flow Measurement Methods Electro-magnetic Flow Measurement (2)

Advantages• Accuracy: Better than 0.5% of measured value• Measuring the velocityvelocity over the entire cross-section• Insensitive to suspended solids

Disadvantages• Re-calibration necessary (every year!!)• The pipe has to be cut open for assembly• Measured value depends on the permeability &

conductivity of the medium • Investment costs increase exponentially to pipe

diameter

Applicable for• Closed conduits ONLY• Gases and liquids with constant conductivity• Pipe diameters up to 600mm (24”)


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Flow Measurement Methods Ultrasonic Flow Measurement (transit time)

Advantages• Accuracy up to 0.5% of measured value

• Medium independent

• No Re-calibration necessary

• Suitable for all types of aquiferall types of aquifer system

• Measurement of meanmean velocityvelocity along the sonic path

• Costs virtually independent of pipe or channel diameter

Disadvantages• Accurate installation required

• Sensitive to suspended solids and air bubbles

• Higher costs for difficult hydraulic conditions, e.g. turbulence

Principle:Measurement of transit time difference between two ultrasonic signals sent with and against the flow. t is proportional to the mean velocity


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Flow Measurement Methods Ultrasonic Flow Measurement (Doppler)

Advantages

• Suitable for all types of aquiferall types of aquifer system

Disadvantages• Low accuracy (3%)

• Flow depends of the medium

• Not qualified for clear waterPrinciple:Measurement of frequency difference between send and receive ultrasonic signals.


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Transit time versus Doppler (1)

Transit time Doppler

time shift of signal frequency shift of signal

fails if particles are present needs particles bigger thancertain size (>l/4) [>0.04mm for water and 1MHz transmitting fre-quency]

fails if air bubbles are present can still work if air bubbles are present

independent of sound velocity dependent of sound velocity

independent of medium temperature dependent on medium temperature


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Transit time Doppler

mean velocity along the path works mainly near the boundaries

weakly dependent on velocity profile dependent on velocity profile

installation distance for proper installation distance for proper

flow conditions: >10D flow conditions: >20D

[D: diameter of pipe] [D: diameter of pipe]

high accuracy possible high accuracy hardly obtainable

signal detection manageable signal detection difficult

if conditions are not optimal if conditions are not optimal

Transit time versus Doppler (2)


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Cost Comparison

0 200 400 600 800 1000 1200 1400

Inve

stm

ent C

osts

Diameter [mm]

Electro-magnetic Flow Measurement

Ultrasonic Flow Measurement


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Basics of ultrasonic transit time method (1)

crucial for a precise v is a precise determination of t (differential transit time)

Q

Sensor 1 (R1)

Sensor 2 (R2)

va

1221

21

12

11

)cos(2

)cos(

)cos(

tt

Lv

vc

Lt

vc

Lt

a

a

a


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c=f(temperature, pressure, salinity)

0 10 20 30 40 50 60 70 80 90 1001400

1420

1440

1460

1480

1500

1520

1540

1560

T[°C]


c[m/s]

T[°C]= inverse of above function in the range of 0 up to 70°

dependency of c for 0 bar and salinity zero


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c

Lt f


Pathlenght Transit timeDifferential time Differential time

v = 0.2 m/s v = 5 m/s[m] [ms] [us] [us]0.5 0.345 0.07 1.751 0.690 0.14 3.502 1.38 0.28 7.005 3.45 0.70 17.5

10 6.90 1.40 35.020 13.8 2.80 70.050 34.5 7.00 175

Water Temp. 20°C c1450m/s


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Determination of path velocity from transit time measurements

Achievable Accuracy transit time measurement

fr

fr

tt

ttLv

cos2

relative error = v/v is dependent on

L, , tf, tr and v

• the larger v, the smaller

• the larger L, the smaller

• errors in L, and t affect more or less linearly

• errors in tf and tr have little effect on for reasonable large tf and tr

RISONIC 2000 Flow Measurment

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